Linear, high density polyethylene (LHDPE) in a "medium" molecular weight range of about 30,000 to about 300,000 (melt index range of about 300 to about 0.01), prepared by low-pressure polymerization of ethylene in the presence of coordination-type catalysts, is a well known, widely used commercial polymer. LHDPE is characterized by having virtually no long-chain branching, short-chain (methyl) branching of less than about 5 CH.sub.3 groups per 1000 carbon atoms, and a density of at least 0.94 g/cm.sup.3. Highly linear LHDPE has a density of at least about 0.96 g/cm.sup.3. It is also well known that LHDPE is crystalline and, after conventional melt-processing, exhibits folded chain crystalline morphology and a melting point in the range of about 128.degree.-137.degree. C. The literature reports many attempts to change the morphology of LHDPE to a more extended chain form by recrystallization at high pressures, resulting in generally higher melting, more crystalline and brittle products. The preparation and properties of these so-called extended chain products have been extensively reviewed; see, for example, Lupton et al. J. Appl. Poly. Sci., 18, 2407 (1974); Geil et al. J. Poly. Sci: A, 2, 3707 (1964).
European Patent Application 89312565.8 (E. I. du Pont de Nemours and Company, published 20 Jun. 1990) discloses heat treatment of ultrahigh molecular weight linear polyethylene (UHMWPE) at 320.degree.-340.degree. C. and atmospheric pressure for at least 30 min, usually about 4 hours, resulting in improved impact resistance, tensile properties and elongation. UHMWPE, as defined in the reference, has a molecular weight of at least 400,000.
European Patent Application 88200202 (Stamicarbon, published 17 Aug. 1988) discloses production of cast film of high density polyethylene of melt index 1-100 wherein the molten polymer is processed at a temperature below 220.degree. C., preferably below 200.degree., more preferably below 180.degree. C.
There are many reports of the annealing of medium molecular weight LHDPE at or below the crystalline melting point to improve crystalline perfection and polymer properties. For example, Schotland et al., Polym. Prepr., Amer. Chem. Soc., Div. Polym Chem., 9 (2), 1219 (1968), describe the effects on certain properties and morphology of annealing blow-molded bottles of high density, medium molecular weight polyethylene at temperatures up to 127.degree. C. Applicant is unaware, however, of any disclosure of LHDPE "annealed" by heating at temperatures close to the decomposition point of the polymer, i.e., in the temperature range of about 250.degree. to about 335.degree. C., or of LHDPE prepared by any process having the improved properties realized in the present invention.